U.S. patent number 10,763,503 [Application Number 15/876,410] was granted by the patent office on 2020-09-01 for composite cathode active material, cathode and lithium battery including the composite cathode active material, and method of preparing the composite cathode active material.
This patent grant is currently assigned to SAMSUNG ELECTRONICS CO., LTD., SAMSUNG SDI CO., LTD.. The grantee listed for this patent is Samsung Electronics Co., Ltd., Samsung SDI Co., Ltd.. Invention is credited to Byungjin Choi, Sukgi Hong, Kanghee Lee, Junho Park, Kwangjin Park, Youhwan Son.
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United States Patent |
10,763,503 |
Park , et al. |
September 1, 2020 |
Composite cathode active material, cathode and lithium battery
including the composite cathode active material, and method of
preparing the composite cathode active material
Abstract
A composite cathode active material, and a cathode and a lithium
battery each including the composite cathode active material. The
composite cathode active material includes: a core including a
first lithium transition metal oxide represented by Formula 1,
Li.sub.aMO.sub.2 wherein, in Formula 1, M includes Ni and at least
one non-nickel Group 4 to Group 13 element, a content of Ni is
about 70 mol % or greater, based on a total content of M,
0.9.ltoreq.a.ltoreq.1.1, and wherein the first lithium transition
metal oxide has a layered crystal structure belonging to an
R.sup.3m space group; and a shell on a surface of the core, the
shell having a spinel crystal structure and including a dopant.
Inventors: |
Park; Junho (Seoul,
KR), Park; Kwangjin (Seongnam-si, KR), Son;
Youhwan (Seongnam-si, KR), Lee; Kanghee
(Suwon-si, KR), Hong; Sukgi (Seongnam-si,
KR), Choi; Byungjin (Seoul, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Samsung Electronics Co., Ltd.
Samsung SDI Co., Ltd. |
Suwon-si, Gyeonggi-do
Yongin-si, Gyeonggi-do |
N/A
N/A |
KR
KR |
|
|
Assignee: |
SAMSUNG ELECTRONICS CO., LTD.
(Gyeonggi-Do, KR)
SAMSUNG SDI CO., LTD. (Gyeonggi-Do, KR)
|
Family
ID: |
63917474 |
Appl.
No.: |
15/876,410 |
Filed: |
January 22, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180316009 A1 |
Nov 1, 2018 |
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Foreign Application Priority Data
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Apr 28, 2017 [KR] |
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10-2017-0055758 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/131 (20130101); H01M 4/525 (20130101); H01M
4/366 (20130101); H01M 4/1391 (20130101); C01D
15/02 (20130101); H01M 10/0525 (20130101); C01P
2002/88 (20130101); H01M 2004/028 (20130101); C01P
2002/01 (20130101); C01P 2002/82 (20130101); C01P
2002/72 (20130101); Y02E 60/10 (20130101); H01M
4/505 (20130101); C01P 2002/32 (20130101); H01M
2300/0088 (20130101) |
Current International
Class: |
H01M
4/52 (20100101); H01M 4/525 (20100101); C01D
15/02 (20060101); H01M 10/0525 (20100101); H01M
4/1391 (20100101); H01M 4/131 (20100101); H01M
4/36 (20060101); H01M 4/02 (20060101); H01M
4/505 (20100101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1020080013822 |
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Feb 2008 |
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KR |
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1020090013841 |
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Feb 2009 |
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KR |
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1020160081692 |
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Jul 2016 |
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KR |
|
Other References
Banerjee et al., "MOF-derived crumpled-sheet-assembled perforated
carbon cuboids as highly effective cathode active materials fro
ultra-high energy density Li-ion hybrid electrochemical capacitors
(Li-HECs)*", Nanoscale, 6, 2014, 4387. cited by applicant .
Peng et al., "Triphenylamine based Metal-Organic Frameworks as
Cathode Materials in Lithium Ion Batteries with Coexistence of
Redox Active sites, High working voltage, and High rate stability",
Applied Materials & Interfaces, 2016, pp. 1-27. cited by
applicant .
Sing et al., "Reporting Physisorption Data for Gas/Solid Systems",
International Union of Pure and Applied Chemistry, vol. 57, No. 4,
1986, pp. 603-619. cited by applicant .
Zhang et al., Monitoring the Solid-state Electrochemistry of
Cu(2,7-AQDC) (AQDC=anthraquinone dicarboxylate) in a Lithium
Battery: Coexistence of Metal and Ligand Redox Activities in a
Metal-Organic Framework, Journal of the American Chemical Society,
2014, p. 1-6. cited by applicant.
|
Primary Examiner: Walke; Amanda C.
Attorney, Agent or Firm: Cantor Colburn LLP
Claims
What is claimed is:
1. A composite cathode active material comprising: a polyhedral
primary particle comprising a core comprising a first lithium
transition metal oxide represented by Formula 1, a shell on a
surface of the core, the shell having a spinel crystal structure
and comprising a dopant, and a polyhedral pore extending from a
first surface to an opposite second surface of the polyhedral
primary particle: Li.sub.aMO.sub.2 Formula 1 wherein, in Formula 1,
M comprises Ni and at least one non-nickel Group 4 to Group 13
element, wherein a content of Ni is about 70 mole percent or
greater, based on a total content of M, 0.9.ltoreq.a.ltoreq.1.1,
and wherein the first lithium transition metal oxide has a layered
crystal structure belonging to an R.sup.3m space group.
2. The composite cathode active material of claim 1, wherein the
dopant comprises at least one non-nickel Group 4 to Group 13
element.
3. The composite cathode active material of claim 1, wherein the
shell comprises a second lithium transition metal oxide represented
by Formula 2: Li.sub.1-xM''.sub.yM'.sub.zO.sub.2 Formula 2 wherein,
in Formula 2, M'' comprises Ni and at least one non-nickel Group 4
to Group 13 element, M' comprises at least one non-nickel Group 4
to Group 13 element, and 0.ltoreq.x.ltoreq.0.05,
0.ltoreq.z.ltoreq.0.06, 1.0.ltoreq.(y+z).ltoreq.1.06.
4. The composite cathode active material of claim 3, wherein M'
comprises Co, Zn, Fe, Cu, Mn, Zr, Ti, Mg, or a combination
thereof.
5. The composite cathode active material of claim 3, wherein the
second lithium transition metal oxide has electrochemical
activity.
6. The composite cathode active material of claim 1, wherein the
shell has a thickness of about 100 nanometers or less.
7. The composite cathode active material of claim 1, wherein a
content of the shell is about 6 weight percent or less, based on a
total weight of the composite cathode active material.
8. The composite cathode active material of claim 1, wherein the
spinel crystal structure belongs to an Fd3m space group.
9. The composite cathode active material of claim 1, wherein a peak
intensity ratio of an intensity of a (003) peak to an intensity of
a (104) peak of the composite cathode active material is less than
a peak intensity ratio of an intensity of a (003) peak to an
intensity of a (104) peak of the core.
10. The composite cathode active material of claim 1, wherein a
maximum peak intensity in a Raman spectrum of the composite cathode
active material is positioned at about 530 inverse centimeters or
greater.
11. The composite cathode active material of claim 1, wherein a
peak intensity ratio of an intensity of a peak at about 530
electron volts to 533 electron volts to an intensity of a peak at
about 528 electron volts to about 530 electron volts in a surface
X-ray photoelectron spectrum of the composite cathode active
material is about 2 or less.
12. The composite cathode active material of claim 1, wherein the
composite cathode active material comprises mesopores having a
diameter of about 1 nanometer to about 100 nanometers, and wherein
the mesopores have an average volume of about 0.001 cubic
centimeters per gram.
13. The composite cathode active material of claim 1, wherein the
composite cathode active material has a specific surface area of
about 0.48 square meters per gram or greater.
14. A composite cathode active material comprising: a primary
particle comprising a core comprising a first lithium transition
metal oxide represented by Formula 1, a shell on a surface of the
core, the shell having a spinel crystal structure and comprising a
dopant, and a polyhedral pore extending from a first surface to an
opposite second surface of the polyhedral primary particle:
Li.sub.aMO.sub.2 Formula 1 wherein, in Formula 1, M comprises Ni
and at least one non-nickel Group 4 to Group 13 element, wherein a
content of Ni is about 70 mole percent or greater, based on a total
content of M, and 0.9.ltoreq.a.ltoreq.1.1, wherein the first
lithium transition metal oxide has a layered crystal structure
belonging to an R.sup.3m space group; and wherein a sum of peak
intensities defined as (Dopant peak intensity+Co peak
intensity)/(Mn peak intensity) before and after Ar.sup.+ sputtering
when analyzed by surface X-ray photoelectron spectroscopy of the
composite cathode active material is about 5 or greater.
15. The composite cathode active material of claim 14, wherein the
dopant comprises at least one non-nickel Group 4 to Group 13
element.
16. The composite cathode active material of claim 14, wherein the
shell comprises a second lithium transition metal oxide represented
by Formula 2: Li.sub.1-xM''.sub.yM'.sub.zO.sub.2 Formula 2 wherein,
in Formula 2, M'' comprises Ni and at least one non-nickel Group 4
to Group 13 element, M' comprises at least one non-nickel Group 4
to Group 13 element, 0.ltoreq.x.ltoreq.0.05,
0.ltoreq.z.ltoreq.0.06, and 1.0.ltoreq.(y+z).ltoreq.1.06.
17. The composite cathode active material of claim 14, wherein M'
comprises Co, Zn, Fe, Cu, Mn, Zr, Ti, Mg, or a combination
thereof.
18. The composite cathode active material of claim 14, wherein the
first lithium transition metal oxide is represented by Formula 3:
Li.sub.aNi.sub.bM1.sub.cM2.sub.dM3.sub.eO.sub.2 Formula 3 wherein,
in Formula 3, M1, M2, and M3 are different and each independently
comprises manganese, vanadium, chromium, iron, cobalt, zirconium,
rhenium, aluminum, boron, germanium, ruthenium, tin, titanium,
niobium, molybdenum, or platinum, 0.9.ltoreq.a.ltoreq.1.1,
0.7<b<1.0, 0<c<0.3, 0<d<0.4, 0.ltoreq.e<0.3,
and b+c+d+e=1.
19. The composite cathode active material of claim 14, wherein the
first lithium transition metal oxide is represented by Formula 4:
Li.sub.aNi.sub.bCo.sub.cMn.sub.dM3'.sub.eO.sub.2 Formula 4 wherein,
in Formula 4, M3' comprises vanadium, chromium, iron, zirconium,
rhenium, aluminum, boron, germanium, ruthenium, tin, titanium,
niobium, molybdenum, or platinum, 0.9.ltoreq.a.ltoreq.1.1,
0.7<b<1.0, 0<c<0.3, 0<d<0.4, 0.ltoreq.e<0.3,
and b+c+d+e=1.
20. A cathode comprising a composite cathode active material
according to claim 1.
21. A lithium battery comprising: the cathode of claim 20, an
anode, and an electrolyte between the cathode and the anode.
22. A method of preparing a composite cathode active material, the
method comprising: mixing a metal-organic framework and a first
lithium transition metal oxide represented by Formula 1 to prepare
a mixture Li.sub.aMO.sub.2 Formula 1 wherein, in Formula 1, M
comprises Ni and at least one non-nickel Group 4 to Group 13
element, a content of Ni is about 70 mole percent or greater, based
on a total content of M, and 0.9.ltoreq.a.ltoreq.1.1, and wherein
the first lithium transition metal oxide has a layered crystal
structure belonging to an R.sup.3m space group; and thermally
treating the mixture under an oxidizing atmosphere at about
650.degree. C. to about 800.degree. C. for about 3 hours to about
20 hours to prepare the composite cathode active material, wherein
the composite cathode active material comprises a polyhedral
primary article comprising: a core comprising the first lithium
transition metal oxide, a shell on a surface of the core, the shell
having a spinel crystal structure and comprising a dopant, and a
polyhedral pore extending from a first surface to an opposite
second surface of the polyhedral primary particle.
23. The method of claim 22, wherein the metal-organic framework
comprises Co, Zn, Fe, Cu, Ni, Mn, Zr, Ti, Mg, or a combination
thereof.
24. The method of claim 22, wherein the metal-organic framework
comprises a polyhedral primary particle, and wherein the polyhedral
primary particle has a particle diameter of about 2 nanometers to
about 300 nanometers.
25. The method of claim 22, wherein an amount of the metal-organic
framework in the mixture is about 6 parts by weight or less, based
on 100 parts by weight of the first lithium transition metal
oxide.
26. The composite cathode active material of claim 14, wherein the
shell is a reaction product of the first lithium oxide and a metal
organic framework.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to and the benefit of Korean
Patent Application No. 10-2017-0055758, filed on Apr. 28, 2017, in
the Korean Intellectual Property Office, and all the benefits
accruing therefrom under 35 U.S.C. .sctn. 119, the content of which
is incorporated herein in its entirety by reference.
BACKGROUND
1. Field
The present disclosure relates to a composite cathode active
material, a cathode, and a lithium battery, each including the
composite cathode active material, and a method of preparing the
composite cathode active material.
2. Description of the Related Art
To the need for devices having a smaller size and higher
performance, it would be desirable to manufacture lithium batteries
that have high energy density, a small size, and a low weight. That
is, lithium batteries of high capacity have become important.
To implement a lithium battery according to the above-described
applications, research has been carried out on cathode active
materials having high capacity. A nickel-based cathode active
material may lead to poor lifetime characteristics and poor thermal
stability due to a side reaction caused by a high content of
residual surface lithium and mixing of cations.
Therefore, there is a need for a method of preventing performance
deterioration in a battery including a nickel-based cathode active
material.
SUMMARY
Provided is a composite cathode active material which may prevent
performance deterioration of a battery by inhibiting a side
reaction on a surface of a composite cathode active material.
Provided is a cathode including the composite cathode active
material.
Provided is a lithium battery including the cathode.
Provided is a method of preparing the composite cathode active
material.
Additional aspects will be set forth in part in the description
which follows and, in part, will be apparent from the description,
or may be learned by practice of the presented embodiments.
According to an aspect of an embodiment, a composite cathode active
material includes: a core including a first lithium transition
metal oxide represented by Formula 1 LiaMO.sub.2 wherein, in
Formula 1, M includes Ni and at least one non-nickel Group 4 to
Group 13 element, wherein a content of Ni is about 70 mole percent
or greater, based on a total content of M, 0.9.ltoreq.a.ltoreq.1.1,
and wherein the first lithium transition metal oxide has a layered
crystal structure belonging to an R.sup.3m space group; and a shell
on a surface of the core, the shell having a spinel crystal
structure and including a dopant.
Also disclosed is composite cathode active material including: a
core including a first lithium transition metal oxide represented
by Formula 1 LiaMO.sub.2 Formula 1 wherein, in Formula 1, M
includes Ni and at least one non-nickel Group 4 to Group 13
element, wherein a content of Ni is about 70 mole percent or
greater, based on a total content of M, and
0.9.ltoreq.a.ltoreq.1.1, wherein the first lithium transition metal
oxide has a layered crystal structure belonging to an R.sup.3m
space group; and a shell on a surface of the core, the shell having
a spinel crystal structure and including a dopant, and wherein a
sum of peak intensities defined as (Dopant peak intensity+Co peak
intensity)/(Mn peak intensity) before and after Ar.sup.+ sputtering
when analyzed by surface X-ray photoelectron spectroscopy of the
composite cathode active material is about 5 or greater.
According to an aspect of an embodiment, a cathode includes the
composite cathode active material.
According to an aspect of an embodiment, a lithium battery includes
the above-described cathode, an anode, and an electrolyte between
the cathode and the anode.
According to an aspect of an embodiment, a method of preparing a
composite cathode active material includes: mixing a metal-organic
framework and a first lithium transition metal oxide represented by
Formula 1 to prepare a mixture LiaMO.sub.2 Formula 1 wherein, in
Formula 1, M includes Ni and at least one non-nickel Group 4 to
Group 13 element, a content of Ni is about 70 mol % or greater,
based on a total content of M, and 0.9.ltoreq.a.ltoreq.1.1, wherein
the first lithium transition metal oxide has a layered crystal
structure belonging to an R.sup.3m space group; and thermally
treating the mixture under an oxidizing atmosphere at about
650.degree. C. to about 800.degree. C. for about 3 hours to about
20 hours to prepare the composite cathode active material.
BRIEF DESCRIPTION OF THE DRAWINGS
These and/or other aspects will become apparent and more readily
appreciated from the following description of the embodiments,
taken in conjunction with the accompanying drawings in which:
FIG. 1A is a scanning electron microscope ("SEM") image of a
surface of a composite cathode active material prepared in Example
4;
FIG. 1B is a transmission electron microscope ("TEM") image of the
surface of the composite cathode active material prepared in
Example 4;
FIG. 1C is a magnified view of a boxed region of FIG. 1B;
FIGS. 1D and 1E are diffraction patterns of boxed regions 3 and 4
of FIG. 1C, respectively;
FIG. 2A is an SEM image of a surface of a composite cathode active
material prepared in Comparative Example 1;
FIG. 2B is a TEM image of the surface of the composite cathode
active material prepared in Comparative Example 1;
FIG. 2C is a magnified view of a boxed region of FIG. 2B;
FIGS. 2D and 2E are diffraction patterns of boxed regions 1 and 2
of FIG. 2C, respectively;
FIGS. 3A to 3C are high-angle annular dark-field ("HAADF") TEM
images of a surface of the composite cathode active material
prepared in Comparative Example 1;
FIGS. 4A to 4C are HAADF TEM images of a surface of the composite
cathode active material prepared in Example 4;
FIG. 5 is a graph of pore volume (cubic centimeters per gram,
cm.sup.3/g) versus pore diameter (nanometers, nm) of the composite
cathode active materials of Examples 1 to 4 and Comparative Example
1;
FIG. 6 is a graph of intensity (arbitrary units, a.u.) versus
diffraction angle (degrees 2-theta, 2.theta.) showing the results
of X-ray diffraction ("XRD") analysis of the composite cathode
active materials of Examples 1 to 4 and Comparative Example 1;
FIG. 7 is a graph of intensity (arbitrary units, a.u.) versus Raman
Shift (inverse centimeters, cm.sup.-1) showing the results of Raman
analysis of the composite cathode active materials of Examples 1, 3
and 4 and Comparative Example 1;
FIG. 8 is a graph of intensity (arbitrary units, a.u.) versus
binding energy (electron volts, eV) showing the results of X-ray
photoelectron analysis of the composite cathode active materials of
Examples 1 to 4 and Comparative Example 1;
FIG. 9 is a graph of heat flow (Watts per gram, W/g) versus
temperature (.degree. C.) showing the results of differential
scanning calorimetry ("DSC") analysis of the composite cathode
active materials of Examples 1 to 4 and Comparative Example 1;
FIG. 10A is a graph of potential (volts versus Li/Li.sup.+) versus
specific capacity (milliampere hours per gram, mAh/g) showing
charge-discharge profiles of a 1.sup.st cycle of the lithium
batteries of Examples 8 to 10 and Comparative Example 4;
FIG. 10B is an enlarged view of FIG. 10A;
FIG. 10C is a graph of differential capacity (dQ/dV) versus voltage
(volts versus Li/Li.sup.+), derived from FIG. 10A; and
FIG. 11 is a schematic view of an exemplary embodiment of a lithium
battery.
DETAILED DESCRIPTION
The present inventive concept will now be described more fully with
reference to the accompanying drawings, in which example
embodiments are shown. The present inventive concept may, however,
be embodied in many different forms, should not be construed as
being limited to the embodiments set forth herein, and should be
construed as including all modifications, equivalents, and
alternatives within the scope of the present inventive concept;
rather, these embodiments are provided so that this inventive
concept will be thorough and complete, and will fully convey the
effects and features of the present inventive concept and ways to
implement the present inventive concept to those skilled in the
art.
The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the inventive concept. As used herein, the singular forms "a", "an"
and "the" are intended to include the plural forms as well, unless
the context clearly indicates otherwise. It will be further
understood that the terms "comprises" and/or "comprising," when
used in this specification, specify the presence of stated
features, integers, steps, operations, elements, and/or components,
but do not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or
groups thereof. As used herein, the slash "/" or the term "and/or"
includes any and all combinations of one or more of the associated
listed items.
In the drawings, the size or thickness of each layer, region, or
element may be exaggerated or reduced for better understanding or
ease of description, and thus the present inventive concept is not
limited thereto. Throughout the written description and drawings,
like or similar reference numbers and labels will be used to denote
like or similar elements. It will also be understood that when an
element such as a layer, a film, a region, or a component is
referred to as being "on" another layer or element, it can be
"directly on" the other layer or element, or intervening layers,
regions, or components may also be present. Although the terms
"first", "second", etc., may be used herein to describe various
elements, components, regions, and/or layers, these elements,
components, regions, and/or layers should not be limited by these
terms. These terms are used only to distinguish one component from
another, not for purposes of limitation.
"About" or "approximately" as used herein is inclusive of the
stated value and means within an acceptable range of deviation for
the particular value as determined by one of ordinary skill in the
art, considering the measurement in question and the error
associated with measurement of the particular quantity (i.e., the
limitations of the measurement system). For example, "about" can
mean within one or more standard deviations, or within .+-.30%,
20%, 10% or 5% of the stated value.
Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
disclosure belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and the present
disclosure, and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
Exemplary embodiments are described herein with reference to cross
section illustrations that are schematic illustrations of idealized
embodiments. As such, variations from the shapes of the
illustrations as a result, for example, of manufacturing techniques
and/or tolerances, are to be expected. Thus, embodiments described
herein should not be construed as limited to the particular shapes
of regions as illustrated herein but are to include deviations in
shapes that result, for example, from manufacturing. For example, a
region illustrated or described as flat may, typically, have rough
and/or nonlinear features. Moreover, sharp angles that are
illustrated may be rounded. Thus, the regions illustrated in the
figures are schematic in nature and their shapes are not intended
to illustrate the precise shape of a region and are not intended to
limit the scope of the present claims.
A C rate is a discharge rate of a cell, and is obtained by dividing
a total capacity of the cell by a total discharge period of time of
1 hour, e.g., a C rate for a battery having a discharge capacity of
1.6 ampere-hours would be 1.6 amperes.
Hereinafter, example embodiments of a composite cathode active
material, a method of preparing the same, and a cathode and a
lithium battery each including the composite cathode active
material will be described in further detail.
According to an embodiment, a composite cathode active material
includes: a core including a first lithium transition metal oxide
represented by Formula 1, the first lithium transition metal oxide
having a layered crystal structure belonging to an R.sup.3m space
group; and a shell on a surface of the core or at least adjacent to
the surface of the core, the shell having a spinel crystal
structure and including a dopant. LiaMO.sub.2 Formula 1
In Formula 1, M may include Ni and at least one non-nickel Group 4
to Group 13 element, a content of Ni may be about 70 mol % or
greater, based on a total content of
M, and 0.9.ltoreq.a.ltoreq.1.1. For example, in Formula 1,
0.95.ltoreq.a.ltoreq.1.05, 1.ltoreq.a.ltoreq.1.04, or
1.ltoreq.a.ltoreq.1.03.
As the composite cathode active material includes the core
including a first lithium transition metal oxide, and the shell
having a spinel crystal structure and including a dopant, a lithium
battery including the composite cathode active material may have
improved charge-discharge characteristics. While not wanting to be
bound by theory, it is understood that the improved
charge-discharge characteristics are due to suppressed
deterioration of the composite cathode active material. For
example, the shell having a spinel crystal structure may be formed
by reaction of the first lithium transition metal oxide and a
metal-organic framework ("MOF"). Since residual lithium on a
surface of the first lithium transition metal oxide serves as a
lithium source during the reaction of the first lithium transition
metal oxide and the MOF to form the shell having a spinel crystal
structure, the resulting composite cathode active material may have
reduced residual lithium content, and a side reaction between the
composite cathode active material and an electrolyte solution may
be suppressed. As the surface of the first lithium transition metal
oxide is coated by the shell having a spinel crystal structure, the
side reaction between the first lithium transition metal oxide and
the electrolyte solution may be effectively inhibited. Since the
shell having a spinel crystal structure, unlike the core having a
layered crystal structure, may provide a 3-dimensional ("3-D")
lithium ion transfer path, a lithium battery including the
composite cathode active material may have a reduced internal
resistance and improved cycle characteristics. Due to the reaction
of the first lithium transition metal oxide and the MOF, the
surface of the first lithium transition metal oxide may be doped
with a metal originating from the MOF, and the layered crystal
structure of the surface of the first lithium transition metal
oxide may be at least partially changed into a spinel crystal
structure, thus forming the shell having the spinel crystal
structure.
In the first lithium transition metal oxide represented by Formula
1 of the composite cathode active material, the Ni content of M,
which includes Ni and at least one non-nickel Group 4 to Group 13
element, may be about 71 mole percent (mol %) or greater, about 75
mol % or greater, about 80 mol % or greater, about 85 mol % or
greater, about 90 mol % or greater, or about 95 mol % or greater,
e.g., about 71 mol % to about 99 mol %, about 75 mol % to about 98
mol %, or about 80 mol % to about 95 mol %, based on a total
content of M. A Ni content of about 70 mol % or greater in the
first lithium transition metal oxide may provide high capacity.
Therefore, a lithium battery having high capacity may be
implemented.
The dopant in the shell having a spinel crystal structure in the
composite cathode active material may be at least one non-nickel
Group 4 to Group 13 element. The dopant may be selected to provide
suitable performance improvement of the composite cathode active
material, and for example, may be a transition metal. The dopant
may include Co, Zn, Fe, Cu, Ni, Mn, Zr, Ti, Mg, or a combination
thereof.
The shell of the composite cathode active material may include a
second lithium transition metal oxide represented by Formula 2.
Li.sub.1-xM''.sub.yM'.sub.zO.sub.2 Formula 2
In Formula 2, M'' may include Ni and at least one non-nickel Group
4 to Group 13 element, M' may be at least one non-nickel Group 4 to
Group 13 element, and 0.ltoreq.x.ltoreq.0.05,
0.ltoreq.z.ltoreq.0.06, 1.0.ltoreq.(y+z).ltoreq.1.06. For example,
in Formula 2, 0.ltoreq.x.ltoreq.0.04, 0.ltoreq.x.ltoreq.0.04,
0.ltoreq.x.ltoreq.0.03, 0.ltoreq.x.ltoreq.0.02, and
0.ltoreq.x.ltoreq.0.01, and in some embodiments,
0.ltoreq.z.ltoreq.0.05, 0.ltoreq.z.ltoreq.0.04,
0.ltoreq.z.ltoreq.0.03, 0.ltoreq.z.ltoreq.0.02, and
0.ltoreq.z.ltoreq.0.01. For example, in Formula 2, the dopant M'
may include Co, Zn, Fe, Cu, Ni, Mn, Zr, Ti, Mg, or a combination
thereof.
The second lithium transition metal oxide of the composite cathode
active material may be electrochemically active. The second lithium
transition metal oxide of the composite cathode active material can
be oxidized or reduced at a potential of 4.5 volts to 4.8 volts
versus Li/Li.sup.+. While not wanting to be bound by theory, it is
understood that because the second lithium transition metal oxide
coated on the layered core is electrochemically active, the
composite cathode active material may have increased discharge
capacity. Due to having a spinel crystal structure, the second
lithium transition metal oxide may provide a 3-D lithium ion
transfer path.
The shell of the composite cathode active material may have a
thickness of about 100 nanometers (nm) or less, about 90 nm or
less, about 80 nm or less, about 70 nm or less, about 60 nm or
less, about 50 nm or less, about 40 nm or less, about 30 nm or
less, about 20 nm or less, or about 10 nm or less. For example, the
shell may have a thickness of about 1 nm or greater or about 5 nm
or greater. In an embodiment, the shell has a thickness of about 1
nm to about 100 nm, for example about 5 nm to about 90 nm. When the
thickness of the shell is within these ranges, a lithium battery
including the composite cathode active material may have further
improved cycle characteristics and thermal stability.
The content of the shell in the composite cathode active material
may be about 6 weight percent (wt %) or less, about 5 wt % or less,
about 4 wt % or less, about 3.5 wt % or less, about 3 wt % or less,
about 2.5 wt % or less, about 2 wt % or less, or about 1 wt % or
less, based on a total weight of the composite cathode active
material. For example, the content of the shell may be about 0.1 wt
% or greater or about 0.5 wt % or greater of the total weight of
the composite cathode active material. In an embodiment, the
content of the shell is about 0.1 wt % to about 6 wt %, or about
0.5 wt % to about 5 wt %. When the content of the shell is within
these ranges, a lithium battery including the composite cathode
active material may have further improved cycle characteristics and
thermal stability.
The spinel crystal structure of the shell may belong to an Fd3m
space group. Since the spinel crystal structure of the shell
belongs to the Fd3m space group, a lithium battery including the
composite cathode active material may have further improved cycle
characteristics and thermal stability.
A peak intensity ratio I(003)/I(104) of a peak of a (003) crystal
plane to a peak of a (104) crystal plane in an X-ray diffraction
("XRD") spectrum of the composite cathode active material including
the core and the shell may be smaller than a peak intensity ratio
I(003)/I(104) of the core including the first lithium transition
metal oxide. In an embodiment, a peak intensity ratio of an
intensity of a (003) peak to an intensity of a (104) peak of the
composite cathode active material may be less than a peak intensity
ratio of an intensity of a (003) peak to an intensity of a (104)
peak of the core. That is, the composite cathode active material
including the core and the shell may have a reduced peak intensity
ratio I(003)/I(104) as compared to a peak intensity ratio
I(003)/I(104) of the first lithium transition metal oxide. Due to
the introduction of the shell having a spinel crystal structure
onto the core having a layered crystal structure, a peak intensity
I(104) of a peak of the (104) crystal plane from the spinel crystal
structure may be relatively increased over a peak intensity I(104)
of a peak obtained from core, i.e., the first lithium transition
metal oxide, leading to a reduced peak intensity ratio
I(003)/I(104) of the composite cathode active material.
A maximum peak value, e.g., intensity, in a Raman spectrum of the
composite cathode active material may be at a wavenumber of about
530 inverse centimeters (cm.sup.-1) or greater, about 532 cm.sup.-1
or greater, about 533 cm.sup.-1 or greater, about 534 cm.sup.-1 or
greater, or about 536 cm.sup.-1 or greater. A maximum peak value in
a Raman spectrum of the core including the first lithium transition
metal oxide may be at about 510 cm.sup.-1. However, due to the
introduction of the shell having a spinel crystal structure onto
the core, a maximum peak shift to about 530 cm.sup.-1 or greater
may occur in the Raman spectrum of the composite cathode active
material.
In a surface X-ray photoelectron ("XPS") spectrum of the composite
cathode active material, a peak intensity ratio
I(530-533)/I(528-530) of a peak at about 530 electron volts (eV) to
533 eV originating from residual surface lithium such as
Li.sub.2CO.sub.3 or LiOH, to a peak at about 528 eV to about 530 eV
originating from oxygen in the crystal structure of the first
lithium transition metal oxide, may be about 2 or less, about 1.9
or less, about 1.8 or less, about 1.7 or less, or about 1.6 or
less. In an embodiment, a peak intensity ratio of an intensity of a
peak at about 530 electronvolts to 533 electronvolts to an
intensity of a peak at about 528 electronvolts to about 530
electronvolts in a surface X-ray photoelectron spectrum of the
composite cathode active material may be about 0.01 to about 2, or
about 0.1 to 1.9. That is, the composite cathode active material
may have reduced residual surface lithium content. Due to the
reduced residual surface lithium content, a side reaction between
the composite cathode active material and an electrolyte may be
inhibited.
Referring to FIGS. 1A to 1E, a composite cathode active material
according to an embodiment may include a polyhedral primary
particle, and the polyhedral primary particle may include a layered
crystal structure and a spinel crystal structure. Referring to
FIGS. 1A to 1E and FIGS. 4A to 4C, a primary particle of the
composite cathode active material may have a polyhedral structure
including a core having the layered crystal structure inside the
primary particle, and a shell coated on the surface of the core,
the shell including a dopant, for example, cobalt (Co), and having
the spinel crystal structure. A polyhedral primary particle is a
primary particle having a plurality of faces and a plurality of
vertices, and is distinguished from primary particles having
spherical or irregular shapes. As shown in FIGS. 1B and 4A, the
polyhedral primary particle may be a primary particle having, for
example, a rectangular cross-sectional shape.
Referring to FIGS. 1B and 4B, the polyhedral primary particle may
include a polyhedral pore as a through-hole. A through-hole refers
to a pore penetrating through opposite faces of the primary
particle. The polyhedral primary particle may include a polyhedral
pore having a rectangular cross-sectional shape, the polyhedral
pore being distinguished from pores having spherical or irregular
cross-sectional shapes.
The composite cathode active material may include a mesopore having
a diameter of about 1 nm to about 100 nm, about 2 nm to about 100
nm, about 5 nm to about 100 nm, about 7 nm to about 100 nm, about
10 nm to about 100 nm, about 10 nm to about 90 nm, about 10 nm to
about 80 nm, about 10 nm to about 70 nm, or about 10 nm to about 60
nm, and an average volume of the mesopore may be about 0.001 cubic
centimeters per gram (cm.sup.3/g) or greater, about 0.002
cm.sup.3/g or greater, or about 0.003 cm.sup.3/g or greater. For
example, the mesopore may have an average volume of about 0.01
cm.sup.3/g or less, about 0.008 cm.sup.3/g or less, or about 0.006
cm.sup.3/g or less. When the composite cathode active material
includes a mesopore having a diameter and an average volume within
these ranges, a lithium battery including the composite cathode
active material may have further improved cycle characteristics and
thermal stability.
Referring to FIG. 1B and FIGS. 4A to 4C, mesopores were found in a
primary particle and among different primary particles.
The composite cathode active material may have a
Brunauer-Emmett-Teller (BET) specific surface area of about 0.48
square meters per gram (m.sup.2/g) or greater, about 0.49 m.sup.2/g
or greater, about 0.50 m.sup.2/g or greater, about 0.52 m.sup.2/g
or greater, as measured by a nitrogen adsorption method. For
example, the composite cathode active material may have a BET
specific surface area of about 1.50 m.sup.2/g or less, about 1.40
m.sup.2/g or less, about 1 m.sup.2/g or less, about 0.80 m.sup.2/g
or less, or about 0.60 m.sup.2/g or less, e.g., about 0.49
m.sup.2/g to about 1.50 m.sup.2/g, or about 0.5 m.sup.2/g to about
1 m.sup.2/g. When the composite cathode active material has a
specific surface area within these ranges, a lithium battery
including the composite cathode active material may have further
improved cycle characteristics and thermal stability.
According to an embodiment, a composite cathode active material
includes: a core including a first lithium transition metal oxide
represented by Formula 1, the first lithium transition metal oxide
having a layered crystal structure belonging to an R.sup.3m space
group; and a shell on a surface of the core or at least adjacent to
the surface of the core, the shell having a spinel crystal
structure and including a dopant, wherein a sum of peak intensities
defined as [(Dopant peak intensity+Co peak intensity)/(Mn peak
intensity)] before and after Ar.sup.+ sputtering in surface X-ray
photoelectron (XPS) spectrum of the composite cathode active
material is about 5 or greater. Li.sub.aMO.sub.2 Formula 1
In Formula 1, M may include Ni and at least one non-nickel Group 4
to Group 13 element, a Ni content in M may be about 70 mol % or
greater, based on a total content of M, and
0.9.ltoreq.a.ltoreq.1.1. For example, in Formula 1,
0.95.ltoreq.a.ltoreq.1.05, 0.97.ltoreq.a.ltoreq.1.03,
0.99.ltoreq.a.ltoreq.1.01, 1.ltoreq.a.ltoreq.1.05,
1.ltoreq.a.ltoreq.1.04, or 1.ltoreq.a.ltoreq.1.03.
In an embodiment wherein the sum of the peak intensities defined as
[(Dopant peak intensity+Co peak intensity)/(Mn peak intensity)]
before and after Ar.sup.+ sputtering in the surface XPS spectrum of
the composite cathode active material is about 5 or greater, a
lithium battery including the composite cathode active material may
have further improved cycle characteristics and thermal stability.
In the composite cathode active material, the shell having the
spinel crystal structure and including a dopant may be the same as
described in the above embodiments.
For example, the first lithium transition metal oxide of the
composite cathode active material may be represented by Formula 3.
Li.sub.aNibM1.sub.cM2.sub.dM3.sub.eO.sub.2 Formula 3
In Formula 3, M1, M2, and M3 may be different and may each
independently include manganese (Mn), vanadium (V), chromium (Cr),
iron (Fe), cobalt (Co), zirconium (Zr), rhenium (Re), aluminum
(Al), boron (B), germanium (Ge), ruthenium (Ru), tin (Sn), titanium
(Ti), niobium (Nb), molybdenum (Mo), or platinum (Pt),
0.9.ltoreq.a.ltoreq.1.1, 0.7<b<1.0, 0<c<0.3,
0<d<0.4, 0.ltoreq.e<0.3, and b+c+d+e=1. For example, in
Formula 3, 0.95.ltoreq.a.ltoreq.1.05, 0.97.ltoreq.a.ltoreq.1.03,
0.99.ltoreq.a.ltoreq.1.01, 1.ltoreq.a.ltoreq.1.05,
1.ltoreq.a.ltoreq.1.04, or 1.ltoreq.a.ltoreq.1.03.
In some embodiments, the first lithium transition metal oxide of
the composite cathode active material may be represented by Formula
4. Li.sub.aNi.sub.bCO.sub.cMn.sub.dM3'.sub.eO.sub.2 Formula 4
In Formula 4, 0.9.ltoreq.a.ltoreq.1.1, 0.7<b<1.0,
0<c<0.3, 0<d<0.4, 0.ltoreq.e<0.3, b+c+d+e=1, and M3'
may be an element selected from vanadium (V), chromium (Cr), iron
(Fe), zirconium (Zr), rhenium (Re), aluminum (Al), boron (B),
germanium (Ge), ruthenium (Ru), tin (Sn), titanium (Ti), niobium
(Nb), molybdenum (Mo), and platinum (Pt). For example, in Formula
4, 0.95.ltoreq.a.ltoreq.1.05, 0.97.ltoreq.a.ltoreq.1.03,
0.99.ltoreq.a.ltoreq.1.01, 1.ltoreq.a.ltoreq.1.05,
1.ltoreq.a.ltoreq.1.04, or 1.ltoreq.a.ltoreq.0.03.
According to an embodiment, a cathode may include the composite
cathode active material according to any of the above-described
embodiments.
The cathode may be prepared as follows. A composite cathode active
material according to any of the above-described embodiments, a
conducting agent, a binder, and a solvent may be mixed together to
prepare a cathode active material composition. The cathode active
material composition may be directly coated on an aluminum current
collector to prepare a cathode plate having a cathode active
material film. In some embodiments, the cathode active material
composition may be cast on a separate support to form a cathode
active material film. This cathode active material film may then be
separated from the support and laminated on an aluminum current
collector to prepare a cathode plate (or a cathode) having the
cathode active material film.
The conducting agent may be carbon black, graphite particulates,
natural graphite, artificial graphite, acetylene black, or Ketjen
black; carbon fibers; carbon nanotubes; a metal powder, metal
fibers, or metal tubes of copper, nickel, aluminum, or silver; or a
conducting polymer such as a polyphenylene derivative, but
embodiments are not limited thereto. Any suitable material
available as a conducting agent in the art may be used.
Examples of the binder are a vinylidene
fluoride/hexafluoropropylene copolymer, polyvinylidene fluoride
("PVDF"), polyacrylonitrile, poly(methyl methacrylate),
polytetrafluoroethylene ("PTFE"), mixtures thereof, and a
styrene-butadiene rubber polymer, but embodiments are not limited
thereto. Any suitable material available as a binding agent in the
art may be used. Examples of the solvent are N-methyl-pyrrolidone
(NMP), acetone, or water, but embodiments are not limited thereto.
Any suitable material available as a solvent in the art may be
used.
In some embodiments, pores may be formed in the cathode plate by
further adding a plasticizing agent to the cathode active material
composition.
The amounts of the composite cathode active material, the
conducting agent, the binder, and the solvent may be the same as
amounts generally used in the art for lithium secondary batteries.
At least one of the conducting agent, the binder, and the solvent
may be omitted according to the use and the structure of the
lithium secondary battery.
The cathode may include a second cathode active material in
addition to the composite cathode active material used above.
The second cathode active material may be any suitable material
available as a cathode active material in the art and, for example,
may be a lithium-containing metal oxide. For example, the second
cathode active material may be a lithium composite oxide including
Co, Mn, Ni, or a combination thereof. In some embodiments, the
second cathode active material may be a compound represented by the
following formulae: Li.sub.aA.sub.1-bB'.sub.bD.sub.2 (wherein
0.90.ltoreq.a.ltoreq.1 and 0.ltoreq.b.ltoreq.0.5);
Li.sub.aE.sub.1-bB'.sub.bO.sub.2-cD.sub.c (wherein
0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.5, and
0.ltoreq.c.ltoreq.0.05); LiE.sub.2-bB'.sub.bO.sub.4-cD.sub.c
(wherein 0.ltoreq.b.ltoreq.0.5 and 0.ltoreq.c.ltoreq.0.05);
Li.sub.aNi.sub.1-b-cCO.sub.bB'.sub.cD.sub..alpha. (wherein
0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, and 0<.alpha.<2);
Li.sub.aNi.sub.1-b-cCo.sub.bB'.sub.cO.sub.2-.alpha.F'.sub..alpha.
(wherein 0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, and 0<.alpha.<2);
Li.sub.aNi.sub.1-b-cCo.sub.bB'.sub.cO.sub.2-.alpha.F'.sub.2
(wherein 0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, and 0<.alpha.<2);
Li.sub.aNi.sub.1-b-cMn.sub.bB'.sub.cD.sub..alpha. (wherein
0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, and 0<.alpha.<2);
Li.sub.aNi.sub.1-b-cMn.sub.bB'.sub.cO.sub.2-.alpha.F'.sub..alpha.
(wherein 0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, and 0.ltoreq.a.ltoreq.2);
Li.sub.aNi.sub.1-b-cMn.sub.bB'.sub.cO.sub.2-.alpha.F'.sub.2
(wherein 0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.5, and 0<.alpha.<2);
Li.sub.aNi.sub.bE.sub.cG.sub.dO.sub.2 (wherein
0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.9,
0.ltoreq.c.ltoreq.0.5, and 0.001.ltoreq.d.ltoreq.0.1);
Li.sub.aNi.sub.bCO.sub.cMn.sub.dG.sub.eO.sub.2 (wherein
0.90.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.0.9,
0.ltoreq.c.ltoreq.0.5, 0.ltoreq.d.ltoreq.50.5, and
0.001.ltoreq.e.ltoreq.0.1); Li.sub.aNiG.sub.bO.sub.2 (wherein
0.90.ltoreq.a.ltoreq.1 and 0.001.ltoreq.b.ltoreq.0.1);
Li.sub.aCoG.sub.bO.sub.2 (wherein 0.90.ltoreq.a.ltoreq.1 and
0.001.ltoreq.b.ltoreq.0.1); Li.sub.aMnG.sub.bO.sub.2 (wherein
0.90.ltoreq.a.ltoreq.1 and 0.001.ltoreq.b.ltoreq.0.1);
Li.sub.aMn.sub.2G.sub.bO.sub.4 (wherein 0.90.ltoreq.a.ltoreq.1 and
0.001.ltoreq.b.ltoreq.0.1); QO.sub.2; QS.sub.2; LiQS.sub.2;
V.sub.2O.sub.5; LiV.sub.2O.sub.5; LiI'O.sub.2; LiNiVO.sub.4;
Li.sub.(3-f)J.sub.2(PO.sub.4).sub.3 (wherein 0.ltoreq.f.ltoreq.2);
Li.sub.(3-f)Fe.sub.2(PO.sub.4).sub.3 (wherein 0.ltoreq.f.ltoreq.2);
and LiFePO.sub.4.
In the formulae above, A may include nickel (Ni), cobalt (Co),
manganese (Mn), or a combination thereof; B' may include aluminum
(Al), nickel (Ni), cobalt (Co), manganese (Mn), chromium (Cr), iron
(Fe), magnesium (Mg), strontium (Sr), vanadium (V), a rare earth
element, or a combination thereof; D may include oxygen (O),
fluorine (F), sulfur (S), phosphorus (P), or a combination thereof;
E may include cobalt (Co), manganese (Mn), or a combination
thereof; F' may include fluorine (F), sulfur (S), phosphorus (P),
or a combination thereof; G may include aluminum (Al), chromium
(Cr), manganese (Mn), iron (Fe), magnesium (Mg), lanthanum (La),
cerium (Ce), strontium (Sr), vanadium (V), or a combination
thereof; Q may include titanium (Ti), molybdenum (Mo), manganese
(Mn), or a combination thereof; I' may include chromium (Cr),
vanadium (V), iron (Fe), scandium (Sc), yttrium (Y), or a
combination thereof; and J may include vanadium (V), chromium (Cr),
manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), or a
combination thereof.
The compounds listed above as cathode active materials may have a
surface coating layer (hereinafter, also referred to as "coating
layer"). In an embodiment, a mixture of a compound without a
coating layer and a compound having a coating layer, the compounds
being those listed above, may be used. In some embodiments, the
coating layer may include an oxide, a hydroxide, an oxyhydroxide,
an oxycarbonate, a hydroxycarbonate, or a combination thereof. In
some embodiments, the compounds for the coating layer may be
amorphous or crystalline. In some embodiments, the coating element
for the coating layer may be magnesium (Mg), aluminum (Al), cobalt
(Co), potassium (K), sodium (Na), calcium (Ca), silicon (Si),
titanium (Ti), vanadium (V), tin (Sn), germanium (Ge), gallium
(Ga), boron (B), arsenic (As), zirconium (Zr), or a combination
thereof. In some embodiments, the coating layer may be formed using
any suitable method that does not adversely affect the physical
properties of the cathode active material when a compound of the
coating element is used. For example, the coating layer may be
formed using a spray coating method or a dipping method. The
coating methods may be well understood by one of ordinary skill in
the art, and thus a detailed description thereof will be
omitted.
According to an embodiment, a lithium battery may include a cathode
including the composite cathode active material according to any of
the above-described embodiments. The lithium battery may be
prepared in the following manner.
First, a cathode may be prepared according to the above-described
method.
Next, an anode may be prepared as follows. The anode may be
prepared in the same manner as applied to the cathode, except that
an anode active material is used instead of the composite cathode
active material. A conducting agent, a binder, and a solvent, which
may be used to prepare an anode active material composition, may be
the same as those used to prepare the cathode.
For example, an anode active material, a conducting agent, a
binder, and a solvent may be mixed together to prepare the anode
active material composition. The anode active material composition
may be directly coated on a copper current collector to prepare an
anode plate (or an anode). In some embodiments, the anode active
material composition may be cast on a separate support to form an
anode active material film. This anode active material film may
then be separated from the support and laminated on a copper
current collector to prepare an anode plate.
The anode active material may be any suitable material that is used
in the art. Examples of the anode active material may include
lithium, a metal alloyable with lithium, a transition metal oxide,
a non-transition metal oxide, a carbonaceous material, and a
combination thereof.
For example, the metal/metalloid alloyable with lithium may be Si,
Sn, Al, Ge, Pb, Bi, Sb, an Si--Y' alloy (wherein Y' may be an
alkali metal, an alkaline earth metal, a Group 13 to 16 element, a
transition metal, a rare earth element, or a combination thereof,
but is not Si), or an Sn--Y' alloy (wherein Y' may be an alkali
metal, an alkaline earth metal, a Group 13 to 16 element, a
transition metal, a rare earth element, or a combination thereof,
but is not Sn). Examples of the element Y' may include Mg, Ca, Sr,
Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc,
Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B,
Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, or a
combination thereof.
Examples of the transition metal oxide may include a lithium
titanium oxide, a vanadium oxide, and a lithium vanadium oxide.
Examples of the non-transition metal oxide may include SnO.sub.2
and SiO.sub.x (wherein 0<x<2).
The carbonaceous material may be crystalline carbon, amorphous
carbon, or a combination thereof. Examples of the crystalline
carbon may include graphite, such as natural graphite or artificial
graphite in non-shaped, plate, flake, spherical, or fibrous form.
Examples of the amorphous carbon may include soft carbon (carbon
calcined at a low temperature), hard carbon, meso-phase pitch
carbonization products, and calcined coke.
The amounts of the anode active material, the conducting agent, the
binder, and the solvent may be the same as amounts generally used
in the art for lithium secondary batteries.
Next, a separator to be disposed between the cathode and the anode
may be prepared. The separator for the lithium battery may be any
suitable separator used in lithium batteries. In some embodiments,
the separator may have low resistance to migration of ions in an
electrolyte and have good electrolyte-retaining ability. Examples
of the separator are glass fiber, polyester, Teflon.TM.,
polyethylene, polypropylene, PTFE, or a combination thereof, each
of which may be a non-woven or woven fabric. For example, a
rollable separator including polyethylene or polypropylene may be
used for a lithium ion battery. A separator with a suitable organic
electrolyte solution-retaining ability may be used for a lithium
ion polymer battery. For example, the separator may be manufactured
in the following manner.
In some embodiments, a polymer resin, a filler, and a solvent may
be mixed together to prepare a separator composition. Then, the
separator composition may be directly coated on an electrode, and
then dried to form the separator. In some embodiments, the
separator composition may be cast on a support and then dried to
form a separator film. This separator film may then be separated
from the support and laminated on an electrode to form the
separator.
The polymer resin used to manufacture the separator may be any
suitable material commonly used as a binder for electrode plates.
Examples of the polymer resin are a
vinylidenefluoride/hexafluoropropylene copolymer, PVDF,
polyacrylonitrile, poly(methylmethacrylate), or a mixture
thereof.
Then, an electrolyte is prepared.
In some embodiments, the electrolyte may be an organic electrolyte
solution. In some embodiments, the electrolyte may be in a solid
phase. Examples of the electrolyte are lithium oxide and lithium
oxynitride. Any suitable material available as a solid electrolyte
in the art may be used. In some embodiments, the solid electrolyte
may be formed on the anode by, for example, sputtering.
In some embodiments, the organic electrolyte solution may be
prepared by dissolving a lithium salt in an organic solvent.
The organic solvent may be any suitable solvent available as an
organic solvent in the art. In some embodiments, the organic
solvent may be propylene carbonate, ethylene carbonate,
fluoroethylene carbonate, butylene carbonate, dimethyl carbonate,
diethyl carbonate, methylethyl carbonate, methylpropyl carbonate,
ethylpropyl carbonate, methylisopropyl carbonate, dipropyl
carbonate, dibutyl carbonate, benzonitrile, acetonitrile,
tetrahydrofuran, 2-methyltetrahydrofuran, .gamma.-butyrolactone,
dioxolane, 4-methyldioxolane, N-dimethyl formamide, dimethyl
acetamide, dimethylsulfoxide, dioxane, 1,2-dimethoxyethane,
sulfolane, dichloroethane, chlorobenzene, nitrobenzene, diethylene
glycol, dimethyl ether, or a mixture thereof.
In some embodiments, the lithium salt may be any suitable material
available as a lithium salt in the art. In some embodiments, the
lithium salt may be LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6,
LiAsF.sub.6, LiClO.sub.4, LiCF.sub.3SO.sub.3,
Li(CF.sub.3SO.sub.2).sub.2N, LiC.sub.4F.sub.9SO.sub.3, LiAlO.sub.2,
LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2)(C.sub.yF.sub.2y+1SO.sub.2) (wherein
x and y may each independently be a natural number), LiCl, LiI, or
a mixture thereof.
Referring to FIG. 11, a lithium battery 1 according to an
embodiment may include a cathode 3, an anode 2, and a separator 4.
In some embodiments, the cathode 3, the anode 2, and the separator
4 may be wound or folded, and then sealed in a battery case 5. In
some embodiments, the battery case 5 may be filled with an organic
electrolyte solution and sealed with a cap assembly 6, thereby
completing the manufacture of the lithium battery 1. In some
embodiments, the battery case 5 may have a cylindrical,
rectangular, or thin-film shape. For example, the lithium battery 1
may be a large-sized thin-film-type battery. In some embodiments,
the lithium battery 1 may be a lithium ion battery.
In some embodiments, the separator 4 may be disposed between the
cathode 3 and the anode 2 to form a battery assembly. In some
embodiments, the battery assembly may be stacked in a bi-cell
structure and impregnated with the electrolyte solution. In some
embodiments, the resultant assembly may be put into a pouch and
hermetically sealed, thereby completing the manufacture of a
lithium ion polymer battery.
In some embodiments, a plurality of battery assemblies may be
stacked to form a battery pack, which may be used in any device
that requires high capacity and high output, for example, in a
laptop computer, a smartphone, or an electric vehicle.
The lithium battery 1 may have improved lifetime characteristics
and high-rate characteristics, and thus may be used in an electric
vehicle ("EV"), for example, in a hybrid vehicle such as a plug-in
hybrid electric vehicle ("PHEV"). The lithium battery may be
applicable to the high-power storage field. For example, the
lithium battery may be used in an electric bicycle or a power
tool.
As the lithium battery is charged to a high voltage of about 4.5
volts (V) with respect to lithium during initial charging, the
spinel crystal structure of the shell (coating layer) belonging to
an Fd3m space group may be activated, generating extra charge
capacity and discharge capacity. Thus, the lithium battery may have
improved initial charge-discharge efficiency.
According to an embodiment, a method of preparing a composite
cathode active material according to any of the above-described
embodiments includes mixing a first lithium transition metal oxide
represented by Formula 1 and a metal-organic framework ("MOF") to
prepare a mixture, the first lithium transition metal oxide having
a layered crystal structure belonging to an R.sup.3m space group;
and thermally treating the mixture under oxidizing atmosphere at
about 650.degree. C. to about 800.degree. C. for about 3 hours to
about 20 hours. Li.sub.aMO.sub.2 Formula 1
In Formula 1, M includes Ni and at least one non-nickel Group 4 to
Group 13 element, a content of Ni is about 70 mol % or greater,
based on a total content of M, and 0.9.ltoreq.a.ltoreq.1.1. For
example, in Formula 1, 0.95.ltoreq.a.ltoreq.1.05,
0.97.ltoreq.a.ltoreq.1.03, 0.99.ltoreq.a.ltoreq.1.01,
1.ltoreq.a.ltoreq.1.05, 1.ltoreq.a.ltoreq.1.04, or
1.ltoreq.a.ltoreq.1.03.
In the method of preparing the composite cathode active material,
after the first lithium transition metal oxide is mixed with the
MOF, the mixture may be thermally treated under an oxidizing
atmosphere as described above to form the shell as a coating layer
on a surface of the first lithium transition metal oxide or in a
region near the surface, the shell having the spinel crystal
structure and including a dopant. The coating layer may be present
in a continuous or discontinuous form on the surface of the core
particle. The shell may fully or partially coat the core
particle.
The thermal treatment temperature may be, for example, about
650.degree. C. to about 800.degree. C., about 650.degree. C. to
about 750.degree. C., or about 700.degree. C. to about 750.degree.
C. The oxidizing atmosphere may include an oxidizing gas, for
example, oxygen or air. The thermal treatment time may be about 3
hours to about 20 hours, about 3 hours to about 15 hours, about 3
hours to about 10 hours, about 3 hours to about 7 hours, or about 4
hours to about 6 hours. However, the thermal treatment conditions
are not limited to these ranges or compositions, and may be
appropriately chosen or varied within ranges allowing the composite
cathode active material to form such a shell having the spinel
crystal structure.
The MOF may include Co, Zn, Fe, Cu, Ni, Mn, Zr, Ti, Mg, or a
combination thereof. The metal of the MOF may be doped on the
surface of the core of the first lithium transition metal oxide or
react with residual surface lithium of the first lithium transition
metal oxide, thus forming the shell. An organic component of the
MOF may be easily removed by decomposition and evaporation at a low
temperature. Accordingly, the metal of the MOF may have relatively
high purity and react with the core of the first lithium transition
metal oxide at a heat treatment temperature of about 650.degree. C.
The relatively high-purity metal of the MOF is distinguished from
other metal sources, for example, a metal salt such as a metal
halide. For example, CoCl.sub.2 has a melting point of about
735.degree. C., similar to the heat treatment temperature.
The MOF may include a polyhedral primary particle. The primary
particle of the MOF may have a polyhedral structure, for example, a
hexahedral, heptahedral, or octahedral structure. The polyhedral
primary particle may have a particle diameter of about 2 nm to
about 300 nm, about 5 nm to about 300 nm, about 10 nm to about 300
nm, about 50 nm to about 300 nm, about 50 nm to about 250 nm, about
50 nm to about 200 nm, or about 100 nm to about 200 nm.
The MOF may be Co-embedded N-doped carbon (Co--NC, for example,
ZIF-67), or Zn-embedded N-doped carbon (Zn--NC, for example,
ZIF-8). However, embodiments are not limited thereto. Any suitable
material available as an MOF in the art may be used.
The amount of the MOF in the mixture obtained by mixing the first
lithium transition metal oxide and the MOF, may be about 6 parts by
weight or less, about 5 parts by weight or less, about 4 parts by
weight or less, or about 3 parts by weight or less, and in some
embodiments, may be about 0.1 parts by weight or greater, or about
0.5 parts by weight or greater, based on 100 parts by weight of the
first lithium transition metal oxide.
An embodiment will now be described in further detail with
reference to the following examples. However, these examples are
only for illustrative purposes and shall not limit the scope of the
disclosed embodiment.
EXAMPLES
Preparation of Metal-Organic Framework (MOF)
Preparation Example 1: Co--NC
1,000 milliliters (mL) of a methanol solution including 600
milliMolar (mM) of 2-methylimidazole was stirred at about 500 rpm
for about 5 minutes. 80 mM of Co(NO.sub.3).sub.2.6H.sub.2O was
added to the methanol solution and stirred at about 500 rpm for
about 12 hours to prepare Co-embedded N-doped carbon (Co--NC,
ZIF-67) nanocrystals. The Co--NC nanocrystals were spun down at
about 5,000 rpm for about 10 minutes. The resulting Co--NC
precipitate was filtered and dried to obtain Co--NC powder. This
Co--NC powder included primary particles having an average particle
diameter of about 100 nm to about 200 nm.
Preparation Example 2: Zn--NC
1,000 mL of a methanol solution including 300 mM of
2-methylimidazole was stirred at about 500 rpm for about 5 minutes.
90 mM of Zn(NO.sub.3).sub.2.6H.sub.2O was added to the methanol
solution and stirred at about 500 rpm for about 12 hours to prepare
Zn-embedded N-doped carbon (Zn--NC, ZIF-8) nanocrystals. The Zn--NC
nanocrystals were spun down at about 5,000 rpm for about 10
minutes. The resulting Zn--NC precipitate was filtered and dried to
obtain Zn--NC powder.
Preparation of Composite Cathode Active Material
Example 1:Ni91+Co--NC 1 wt % by Drying Process
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 was added into distilled
water, followed by filtration, drying at about 120.degree. C.
(i.e., washing process).
100 parts by weight of the washed
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide and 1 part by weight of the Co--NC powder as
the MOF prepared in Preparation Example 1 were mixed to prepare a
mixture.
This mixture was put into a furnace and then thermally treated at
about 720.degree. C. for about 5 hours while oxygen was flowed into
the furnace.
During the thermal treatment process, a coating layer having a
Co-doped spinel crystal structure was formed on the
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 core by reaction of
residual surface lithium such as Li.sub.2CO.sub.3 and LiOH on the
core surface with the Co--NC.
During the thermal treatment process, the organic component of the
Co--NC was easily removed by evaporation.
Example 2:Ni91+Co--NC 2 wt % by Drying Process
A composite cathode active material was prepared in the same manner
as in Example 1, except that 100 parts by weight of the washed
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide and 2 parts by weight of the Co--NC powder
prepared as the MOF in Preparation Example 1 were mixed to prepare
a mixture.
Example 3: Ni91+Co--NC 3 wt % by Drying Process
A composite cathode active material was prepared in the same manner
as in Example 1, except that 100 parts by weight of the washed
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide and 3 parts by weight of the Co--NC powder
prepared as the MOF in Preparation Example 1 were mixed to prepare
a mixture.
Example 4: Ni91+Co--NC 6 wt % by Drying Process
A composite cathode active material was prepared in the same manner
as in Example 1, except that 100 parts by weight of the washed
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide and 6 parts by weight of the Co--NC powder
prepared as the MOF in Preparation Example 1 were mixed to prepare
a mixture.
Example 5: Ni91+Zn--NC 1 wt % by Drying Process
A composite cathode active material was prepared in the same manner
as in Example 1, except that 100 parts by weight of the washed
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide and 1 part by weight of the Zn--NC powder
prepared as the MOF in Preparation Example 2 were mixed to prepare
a mixture.
Example 6: Ni91+(Co--NC+Zn--NC) 1 wt % by Drying Process
A composite cathode active material was prepared in the same manner
as in Example 1, except that 100 parts by weight of the washed
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide, 0.5 parts by weight of the Co--NC powder
prepared as the MOF in Preparation Example 1, and 0.5 parts by
weight of the Zn--NC powder prepared as the MOF in Preparation
Example 2 were mixed to prepare a mixture.
During the thermal treatment process, a coating layer having a Co-
and Zn-doped spinel crystal structure was formed on the
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 core by reaction of
residual surface lithium such as Li.sub.2CO.sub.3 and LiOH on the
surface of the core with the Co--NC and Zn--NC.
Example 7: Ni80+Co--NC 1 wt % by Drying Process
100 parts by weight of unwashed
LiNi.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide and 1 part by weight of the Co--NC powder as
the MOF prepared in Preparation Example 1 were mixed to prepare a
mixture.
This mixture was put into a furnace and then thermally treated at
about 720.degree. C. for about 5 hours while oxygen was flowed into
the furnace.
During the thermal treatment process, a coating layer having a
Co-doped spinel crystal structure was formed on the
LiNi.sub.0.80Co.sub.0.15Mn.sub.0.05O.sub.2 core by reaction of
residual surface lithium such as Li.sub.2CO.sub.3 and LiOH on the
surface of the core with the Co--NC.
Comparative Example 1: Ni91 Alone (Bare--without Washing)
Unwashed LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first
lithium transition metal oxide was itself used as a composite
cathode active material.
Comparative Example 2: Ni91 Alone (Washing)
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide, washed in the same manner as in Example 1,
was itself used as a composite cathode active material.
Comparative Example 3: Ni91+CoCl.sub.2 0.75 wt % by Drying
Process
100 parts by weight of unwashed
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 as a first lithium
transition metal oxide and 0.75 parts by weight of CoCl.sub.2 were
added into distilled water, followed by filtration and drying at
about 120.degree. C. to obtain a dried product. This dried product
was put into a furnace and then thermally treated at about
750.degree. C. for about 10 hours while oxygen was flowed into the
furnace, thereby preparing a composite cathode active material
including a Co-doped coating layer on the
LiNi.sub.0.91Co.sub.0.04Mn.sub.0.05O.sub.2 core.
Manufacture of Lithium Battery (Half Cell)
Example 8
Manufacture of Cathode
The composite cathode active material prepared in Example 1, a
carbon conducting material (Denka Black), and polyvinylidene
fluoride (PVdF) were mixed at a weight ratio of 92:4:4 to prepare a
mixture, and the mixture was mixed with N-methylpyrrolidone (NMP)
in an agate mortar to prepare a slurry. The slurry was bar-coated
on an aluminum current collector having a thickness of 15 .mu.m,
dried at room temperature, dried once more in a vacuum at
120.degree. C., and then roll-pressed and punched to manufacture a
cathode plate having a thickness of 55 .mu.m.
Manufacture of Coin Cell
A coin cell was manufactured using the cathode plate manufactured
above, lithium metal as a counter electrode, a PTFE separator, and
a solution prepared as an electrolyte by dissolving 1.25 Molar (M)
of LiPF.sub.6 in a mixture of ethylene carbonate ("EC"),
ethylmethyl carbonate ("EMC"), and dimethyl carbonate ("DMC") at a
volume ratio of 3:4:3.
Examples 9 to 14
Coin cells were manufactured in the same manner as in Example 8,
except that the composite cathode active materials prepared in
Examples 2 to 7 were used, respectively, instead of the composite
cathode active material prepared in Example 1.
Comparative Examples 4 to 6
Coin cells were manufactured in the same manner as in Example 8,
except that the composite cathode active materials prepared in
Comparative Examples 1 to 3 were used, respectively, instead of the
composite cathode active material prepared in Example 1.
Evaluation Example 1: Surface Composition Evaluation
Surfaces of the composite cathode active materials prepared in
Comparative Example 1 and Examples 1 to 4 were analyzed by
inductively coupled plasma spectroscopy ("ICP") and X-ray
photoelectron spectroscopy ("XPS"). The results are shown in Tables
1 and 2.
TABLE-US-00001 TABLE 1 Molar ratio (Ni + Co + Mn = 1 mol) Example
Li Mn Co Ni Comparative 1.03 0.050 0.149 0.802 Example 1 Example 1
1.01 0.048 0.150 0.802 Example 2 1.08 0.050 0.157 0.793 Example 3
1.07 0.049 0.160 0.791 Example 4 1.09 0.048 0.167 0.784
TABLE-US-00002 TABLE 2 Sum of intensity ratios of Co peak to Mn
peak Example before and after Ar.sup.+ sputtering Comparative
Example 1 4.7 Example 1 5.6 Example 2 6.5 Example 3 6.5 Example 4
9.9
Referring to Table 1, with the increasing Co--NC coating amounts in
Examples 1 to 4, the Co--NC coated composite cathode active
materials of Examples 1 to 4 had increased amounts of doped Co on
the surfaces thereof, compared to the composite cathode active
material of Comparative Example 1 including no coated Co--NC.
Table 2 is a table of a sum of an intensity ratio of a Co peak to a
Mn peak ([Co peak intensity/Mn peak intensity]) in an XPS spectrum
of each of the composite cathode active materials of Comparative
Example 1 and Examples 1 to 4 before the surface of each of the
composite cathode active materials was etched by Ar.sup.+ ion
sputtering, and an intensity ratio of a Co peak to a Mn peak in an
internal XPS spectrum of each of the corresponding composite
cathode active materials after the surface of each of the composite
cathode active materials was etched by Ar.sup.+ ion sputtering. The
Ar.sup.+ ion sputtering was performed onto an area of 1 millimeter
squared (mm.sup.2) at a voltage of about 1 kV for about 2
minutes.
Referring to Table 2, in the composite cathode active materials of
Examples 1 to 4, the sum of the intensity ratios of a Co peak to a
Mn peak before and after etching of the surface of each composite
cathode active material was 5 or greater, indicating an increased
Co content, originating from the Co--NC, on the surface of each
composite cathode active material, compared to the composite
cathode active material of Comparative Example 1. It was found that
a Co-doped coating layer is formed on the surface of core, i.e.,
the composite cathode active material of Comparative Example 1, in
Examples 1 to 4.
Evaluation Example 2: Surface Crystal Structure Evaluation
As shown in FIG. 1A, which is a scanning electron microscope
("SEM") image of the composite cathode active material prepared in
Example 4, the composite cathode active material of Example 4
included secondary particles resulting from aggregation of a
plurality of primary particles.
As shown in FIG. 1B, which is a transmission electron microscope
("TEM") image of the composite cathode active material of Example
4, a primary particle of a secondary particle had a polyhedral
structure having a rectangular cross-sectional shape and included a
through-hole as a polyhedral pore having a rectangular
cross-sectional shape. In FIG. 1B, the through-hole appears as a
bright region inside a polygonal dark (black) primary particle.
Referring to FIG. 1C, which is a magnified view of a portion of
FIG. 1B, in the primary particle of the composite cathode active
material prepared in Example 4, an inner region 3 thereof was found
to have a layered crystal structure and a surface region 4 thereof
was found to have a spinel crystal structure. In FIGS. 1D and 1E,
which are magnified views of the inner and surface regions of the
primary particle, respectively, the layered crystal structure and
the spinel crystal structure are shown, respectively. On the
surface of the core of the composite cathode active material of
Example 4 having the layered crystal structure, a coating layer
having the Co-doped spinel crystal structure was formed. The
coating layer had a thickness of about 10 nm. It was also found
that the spinel crystal structure belongs to an Fd3m space
group.
As shown in FIG. 2A, which is an SEM image of the composite cathode
active material prepared in Comparative Example 1, the composite
cathode active material of Comparative Example 1 included secondary
particles resulting from aggregation of a plurality of primary
particles.
As shown in FIG. 2B, which is a TEM image of the composite cathode
active material of Comparative Example 1, a primary particle of a
secondary particle had a polyhedral structure having a rectangular
cross-sectional shape, without a through-hole.
Referring to FIG. 2C, which is a magnified view of a portion of
FIG. 2B, the primary particle of the composite cathode active
material prepared in Comparative Example 1 was found to have a
layered crystal structure in both an inner region 1 and a surface
region 2 thereof. In FIGS. 2D and 2E, which are magnified views of
the inner and surface regions, respectively, of the primary
particle, the layered crystal structures are shown. That is, on the
surface of the core of the composite cathode active material of
Comparative Example 1 having the layered crystal structure, a
coating layer having a spinel crystal structure was not formed.
Evaluation Example 3: Surface Composition Evaluation
FIGS. 3A to 3C are high-angle annular dark-field ("HAADF") TEM
images of the composite cathode active material of Comparative
Example 1 including aggregates of a plurality of primary
particles.
FIGS. 4A to 4C are HAADF TEM images of the composite cathode active
material of Example 4 including aggregates of a plurality of
primary particles.
As shown in FIG. 4A, the composite cathode active material of
Example 4 included more pores, including pores in the form of
through-holes, in the primary particles, compared to the composite
cathode active material of Comparative Example 1 shown in FIG.
3A.
Compared to the composite cathode active material of Comparative
Example 1 as shown in FIG. 3B, the composite cathode active
material of Example 4 was found to include more pores in the
primary particles with surfaces uniformly doped with oxygen, as
shown in an oxygen-mapping image of FIG. 4B.
Compared to the composite cathode active material of Comparative
Example 1 shown in FIG. 3C, the composite cathode active material
of Example 4 was found to include more pores in the primary
particles with surfaces uniformly doped with cobalt (Co), as shown
in a Co-mapping image of FIG. 4C.
Therefore, the composite cathode active material of Example 4 was
found to have a Co-doped coating layer as a shell on its
surface.
Evaluation Example 4: Specific Surface Area and Mesopore Size
Evaluation
Brunauer-Emmett-Teller ("BET") specific surface areas of the
composite cathode active materials prepared in Comparative Example
1 and Examples 1 to 4 were analyzed by a nitrogen adsorption
method. The results are shown in Table 3. A distribution of pore
volume with respect to pore size in each of the composite cathode
active materials is shown in FIG. 5.
TABLE-US-00003 TABLE 3 Example Specific surface area (m.sup.2/g)
Comparative Example 1 0.47954 Example 1 0.48603 Example 2 0.52289
Example 3 0.67593 Example 4 1.4141
As shown in Table 3, the composite cathode active materials of
Examples 1 to 4 had an increased specific surface area, which was
further increased with increased amounts of coated Co--NC, as
compared to the composite cathode active material of Comparative
Example 1.
As shown in FIG. 5, the composite cathode active materials of
Examples 2 to 4 were found to have larger mesopores having a pore
diameter of about 10 nm to about 100 nm and a pore volume of about
0.001 cm.sup.3/g to about 0.005 cm.sup.3/g.
Evaluation Example 5: X-Ray Diffraction ("XRD") Analysis
As shown in FIG. 6, in the XRD spectra of the composite cathode
active materials of Examples 1 to 4, a peak intensity ratio
I(003)/I(104) of the intensity of a peak of the (003) crystal plane
to the intensity of a peak of the (104) crystal plane was smaller
than a peak intensity ratio I(003)/I(104) in the XRD spectrum of
the composite cathode active material of Comparative Example 1.
That is, due to the inclusion of a pyrolysis product of the MOF
coated on the composite cathode active material of Comparative
Example 1 having a layered crystal structure, the composite cathode
active materials of Examples 1 to 4 had a reduced peak intensity
ratio I(003)/I(104) of the peak of the (003) crystal plane to the
peak of the (104) crystal plane.
Such a reduced peak intensity ratio I(003)/I(104) of the peak of
the (003) crystal plane at about 19.degree. to the peak of the
(104) crystal plane at about 44.degree. in the XRD spectrum of the
lithium transition metal oxide having a layered crystal structure
may indicate a reduced size of crystals having the layered crystal
structure, a reduced amount of the layered crystal structure, and
formation of a spinel crystal structure.
Evaluation Example 6: Raman Analysis
Referring to FIG. 7, a maximum peak value near 510 cm.sup.-1 in the
Raman spectrum of the composite cathode active material of
Comparative Example 1 was shifted to near 530 cm.sup.-1 or greater
in the Raman spectra of the composite cathode active materials of
Examples 1, 2, and 3. This maximum peak value shift indicates that
a coating layer having a spinel phase was formed on the core of the
composite cathode active material.
Evaluation Example 7: Residual Surface Lithium Content Evaluation
by XPS
Residual Surface lithium contents of the composite cathode active
materials of Comparative Example 1 and Examples 1 to 4 were
analyzed by XPS. The results are shown in FIG. 8 and Table 4.
In FIG. 8, lower XPS spectra are the results of surface composition
analysis of the composite active materials before sputtering, and
upper XPS spectra are the results of inner composition analysis of
the composite active materials after sputtering to etch away the
surface of the composite active materials until their inside was
exposed. The sputtering was performed by Ar.sup.+ ion sputtering
onto an area of about 1 mm.sup.2 for about 2 minutes with a voltage
of 1 kV.
In the surface XPS spectra before sputtering in FIG. 8, peaks at
about 530 eV to about 533 eV originate from residual lithium such
as Li.sub.2CO.sub.3 or LiOH, while peaks at about 528 eV to about
530 eV originate from oxygen (O.sub.lattice) in the layered or
spinel crystal structure and are unrelated to residual lithium.
TABLE-US-00004 TABLE 4 Peak intensity Peak intensity ratio Peak
intensity ratio ratio difference
(I(Li.sub.2CO.sub.3)/I(O.sub.lattice))
(I(Li.sub.2CO.sub.3)/I(O.sub.latti- ce)) before and after Example
before sputtering after sputtering sputtering Comparative 2.24 0.90
1.34 Example 1 Example 1 1.62 0.93 0.69 Example 2 1.19 0.76 0.43
Example 3 1.10 0.77 0.33 Example 4 0.62 0.84 -0.22
As shown in Table 4 and FIG. 8, the composite cathode active
materials of Examples 1 to 4 had a reduced peak intensity ratio
(I(Li.sub.2CO.sub.3)/I(O.sub.lattice)) before sputtering, as
compared to the composite cathode active material of Comparative
Example 1, indicating reduction in residual surface lithium
content. This is attributed to the formation of the coating layer
by reaction of Co--NC with the residual lithium. A difference in
peak intensity ratio (I(Li.sub.2CO.sub.3)/I(O.sub.lattice)) before
and after sputtering, i.e., a difference between the residual
surface lithium content and the inner residual lithium content of
the composite cathode active material was significantly smaller in
the composite cathode active materials of Examples 1 to 4, compared
to the composite cathode active material of Comparative Example
1.
Evaluation Example 8: Thermal Stability Evaluation
Thermal stabilities of the composite cathode active materials of
Comparative Example 1 and Examples 1 to 4 were evaluated by
differential scanning calorimetry (DSC). The results are shown in
Table 5 and FIG. 9.
TABLE-US-00005 TABLE 5 Example Exothermic calorific value (J/g)
Comparative Example 1 2160 Example 1 2147 Example 2 1975 Example 3
1901 Example 4 1511
Referring to Table 5, the composite cathode active materials of
Examples 1 to 4 were found to have a reduced exothermic calorific
value, as compared to the composite cathode active material of
Comparative Example 1.
Referring to FIG. 9, the composite cathode active materials of
Examples 1 to 4 were found to have a smaller low-temperature
exothermic peak near about 225.degree. C., with an exothermic peak
shift at higher temperatures, indicating improved thermal stability
as compared with the composite cathode active material of
Comparative Example 1.
Evaluation Example 9: Charge-Discharge Characteristics
Evaluation
The lithium batteries of Examples 8 to 14 and Comparative Examples
4 to 6 were charged at about 25.degree. C. with a constant current
("CC") of 0.1 C rate until a voltage of 4.35 V (with respect to Li)
was reached, and then with a constant voltage of 4.35 V (constant
voltage mode) until a cutoff current of 0.05 C rate was reached,
followed by discharging with a constant current of 0.1 C rate until
a voltage of 2.8 V (with respect to Li) was reached (1.sup.st
cycle, formation cycle).
After the 1.sup.st cycle, the lithium batteries were charged at
about 25.degree. C. with a constant current of 0.33 C rate until a
voltage of 4.35 V (with respect to Li) was reached, and then with a
constant voltage of 4.35 V (constant voltage mode) until a cutoff
current of 0.05 C rate was reached, followed by discharging with a
constant current of 0.2 C rate until a voltage of 2.8 V (with
respect to Li) was reached (2.sup.nd cycle).
After the 2.sup.nd cycle, the lithium batteries were charged at
about 25.degree. C. with a constant current of 0.33 C rate until a
voltage of 4.35 V (with respect to Li) was reached, and then with a
constant voltage of 4.35 V (constant voltage mode) until a cutoff
current of 0.05 C rate was reached, followed by discharging with a
constant current of 0.2 C rate until a voltage of 2.8 V (with
respect to Li) was reached. This cycle was repeated to the
60.sup.th cycle. A rest time of about 10 minutes was allowed after
each charge/discharge cycle among all of the charge/discharge
cycles.
The charge-discharge test results are shown in Table 6. A capacity
retention at the 50.sup.th cycle and aninitial charge-discharge
efficiency are defined as shown in Equations 1 and 2, respectively.
Capacity retention (%)=[Discharge capacity at 50.sup.th
cycle/Discharge capacity at 1.sup.st cycle].times.100% Equation 1
Initial charge-discharge efficiency (%)=[Discharge capacity at
1.sup.st cycle/Charge capacity at 1.sup.st cycle].times.100%
Equation 2
TABLE-US-00006 TABLE 6 Initial charge- Residual Charge discharge
Discharge Capacity surface capacity at efficiency capacity at
retention lithium Example 1.sup.st cycle (mAh/g) (%) 2.sup.nd cycle
(mAh/g) (%) content (ppm) Comparative 239 90 217 82.3 3,556 Example
4 Comparative 247 92 224 87.8 1,202 Example 5 Comparative 248 92
224 82.4 1,592 Example 6 Example 8 249 92 225 91.2 847 Example 9
248 91 221 91.2 466 Example 12 248 90 221 91.9 1,104 Example 13 248
91 222 92.1 1,075 Example 14 243 92 223 84.3 3,767
Referring to Table 6, the lithium batteries as coin cells of
Examples 8 to 13, which included the composite cathode active
material prepared through washing and a pyrolysis product of Co--NC
coated on the surface thereof, were each found to have a remarkably
improved capacity retention and a reduced residual surface lithium
content, as compared to the lithium battery of Comparative Example
4 including the composite cathode active material prepared without
washing and such a coating layer, the lithium battery of
Comparative Example 5 including the composite cathode active
material prepared through washing but having no coating layer, and
the lithium battery of Comparative Example 6 including the
composite cathode active material of which a surface was doped
and/or coated with Co in a wet manner.
The lithium battery of Example 14 including the composite cathode
active material prepared without washing and including a pyrolysis
product of Co--NC coated on the surface thereof was found to have
an improved capacity retention, as compared with the lithium
battery of Comparative Example 4 including the composite cathode
active material prepared without washing and such a coating.
In the lithium batteries of Comparative Examples 4 to 6, since the
composite cathode active material includes only a layered crystal
structure, 2-dimensional lithium ion transfer occurs in the layered
crystal structure. However, in the lithium batteries of Examples 8
to 13, the composite cathode active material further included a
shell as a coating layer on the layered crystal core, the shell
having a spinel crystal structure with a 3-D lithium ion transfer
pathway. A composite cathode active material further including a
coating layer having a spinel crystal structure on the surface of a
core having a layered crystal structure may provide an improved
lithium ion transfer pathway, compared to a composite cathode
active material including only the layered crystal structure.
Accordingly, the lithium batteries of Examples 8 to 13 had improved
charge-discharge characteristics, compared to the lithium batteries
of Comparative examples 4 to 6, as shown in Table 6. That is, the
shell as a coating layer having the spinel crystal structure may
serve as a lithium ion conductor providing a lithium ion transfer
pathway. The shell as a coating layer having the spinel crystal
structure may have electrochemical activity, and thus may provide
additional improved capacity.
Evaluation Example 10: Charge-Discharge Characteristics
Evaluation
The lithium batteries of Examples 8 to 10 and Comparative Example 4
were charged at about 25.degree. C. with a constant current of 0.1
C rate until a voltage of 4.8 V (with respect to Li) was reached,
and then with a constant voltage of 4.8 V (constant voltage mode)
until a cutoff current of 0.05 C rate was reached, followed by
discharging with a constant current of 0.1 C rate until a voltage
of 2.0 V (with respect to Li) was reached (1.sup.st cycle).
After the 1.sup.st cycle were, the lithium batteries charged at
about 25.degree. C. with a constant current of 0.33 C rate until a
voltage of 4.8 V (with respect to Li) was reached, and then with a
constant voltage of 4.8 V (constant voltage mode) until a cutoff
current of 0.05 C rate was reached, followed by discharging with a
constant current of 0.2 C rate until a voltage of 2.0 V (with
respect to Li) was reached. This cycle was repeated to the
60.sup.th cycle. A rest time of about 10 minutes was allowed after
each charge/discharge cycle among all the charge/discharge
cycles.
The charge-discharge test results are shown in Table 7. The
capacity retention at the 50.sup.th cycle and the initial
charge-discharge efficiency are as defined in Equations 1 and 2,
respectively.
Charge-discharge profiles at the 1.sup.st cycle are shown in FIGS.
10A and 10B.
TABLE-US-00007 TABLE 7 Initial charge- Discharge Charge capacity
discharge capacity at Capacity at efficiency 2.sup.nd cycle
retention Example 1.sup.st cycle (mAh/g) (%) (mAh/g) (%)
Comparative 264 90 222 83.7 Example 4 Example8 269 91 225 87.7
Referring to Table 7, the lithium battery of Example 8 was found to
be improved in terms of charge capacity, discharge capacity,
initial efficiency, and capacity retention, as compared to the
lithium battery of Comparative Example 4.
Referring to FIGS. 10A and 10B, as compared to the lithium battery
of Comparative Example 4, at a voltage of 4.5 V or greater during
the 1.sup.st cycle of charging to 4.8V, additional charge capacity
was exhibited in the lithium batteries of Examples 8 to 10 due to
the coating layer or shell region having the spinel crystal
structure.
Consequently, this also led to additional discharge capacity in the
lithium batteries of Examples 8 to 10. To more clearly support this
result, the charge-discharge profile of FIG. 10A was represented as
a plot of dQ/dV with respect to voltage (V) in FIG. 10C, wherein
dQ/dV represents a differential value of charge quantity (Q) with
respect to voltage (V). Referring to FIG. 10C, a peak due to the
additional capacity exhibited by the coating layer or shell having
the spinel crystal structure appeared at a voltage of about 4.5V or
greater, indicating that the coating layer or shell region having
the spinel crystal structure has electrochemical activity.
As described above, according to the one or more embodiments, using
a composite cathode active material including a core and a shell on
the core, the shell having a metal-doped spinel crystal structure,
a lithium battery may have improved charge-discharge
characteristics and improved thermal stability.
It should be understood that embodiments described herein should be
considered in a descriptive sense only and not for purposes of
limitation. Descriptions of features or aspects within each
embodiment should typically be considered as available for other
similar features or aspects in other embodiments.
While one or more embodiments have been described with reference to
the figures, it will be understood by those of ordinary skill in
the art that various changes in form and details may be made
therein without departing from the spirit and scope as defined by
the following claims.
* * * * *